app: generate proposer config file for validator clients

[KaloyanTanev]

Generates a proposer-config.json file in a vc-config directory next to the cluster lock on startup: created if missing, never modified afterwards. The file uses a charon-defined canonical schema (version: 1, modelled on the proposer settings format minus its pre-gloas legacy fields): default_config carries the per-field majority fee_recipient and gas_limit across this node's validator shares (from charon's effective builder registrations, including operator overrides, falling back to the lock), and proposer_config only lists validators diverging from it, carrying only the diverging fields. Per validator client adapters (the CDVN wrapper scripts) render each client's native format from it.

From the gloas fork, validator clients sign the fee recipient and gas limit into SignedProposerPreferences, so VCs must be configured with the cluster values instead of relying on charon overriding them (see #4692 for the full design). Generation is best-effort: failures are logged and never prevent duties. Also adds a GasLimit accessor to BuilderRegistrationService and rejects builder registration overrides with a zero gas limit.

category: feature
ticket: #4692

[codecov]

Codecov Report

❌ Patch coverage is 81.63265% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.53%. Comparing base (2ab39d6) to head (8330ed0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
app/proposerconfig.go	83.33%	13 Missing ⚠️
app/builderregistration.go	78.94%	4 Missing ⚠️
app/app.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4700      +/-   ##
==========================================
+ Coverage   65.52%   65.53%   +0.01%     
==========================================
  Files         246      247       +1     
  Lines       30062    30158      +96     
==========================================
+ Hits        19698    19765      +67     
- Misses      10363    10392      +29     
  Partials        1        1              

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud